1. Field of the Invention
This invention relates to a method and apparatus for ablative pyrolysis of biomass to form condensable liquid products having properties, such as viscosity and energy density, that make the products useful as fuel oil, or as a source of chemicals, or for the production of chemicals and/or derived products.
2. Description of Related Art
In general, pyrolysis is a chemical process in which a compound (feedstock) is converted to one or more products by heat. Most pyrolysis processes utilize heat transfer from a hot gas and/or hot solid, such as sand, to the feedstock and rely upon the particles of the feedstock being small to achieve rapid heating. This process is typically carried out in entrained flow, transported, fluid, or circulating beds.
Fast pyrolysis is a process in which a solid feedstock, such as wood, agricultural waste, or other organic based material is rapidly heated, producing a solid product and vapor, the latter of which may then be condensed to form a liquid product. In the fast pyrolysis of biomass, which is generally defined as renewable organic materials, such as wood, agricultural crops or wastes, and municipal wastes, the liquid product is generally referred to as bio-oil, biocrude, and pyrolysis oil, although other terms also occur in the literature. This liquid product, which will be referred to herein as pyrolysis oil, is a complex liquid comprising water and a wide range of hydrocarbons and carbohydrates produced by the thermal breakdown of lignin, cellulose, and hemicellulose in the biomass. Recent studies have shown that pyrolysis oil may function as a substitute for oils derived from petroleum in certain applications, and can be upgraded using existing techniques to produce products similar to gasoline, diesel fuel, and heavy oil, with high overall thermal efficiencies (on the order of 50% for gasoline-range hydrocarbon products). Fast pyrolysis is distinct from slow pyrolysis and is characterized by very rapid heating of the feedstock followed by rapid cooling and condensation of the resulting vapor stream. Fast pyrolysis has been shown to convert as much as 75% by mass of the original biomass into pyrolysis oil, with the remainder being converted into non-condensable gases and a solid product, i.e. primarily char. On the other hand, slow pyrolysis produces a greater proportion of solid product and involves much slower heating of the feedstock. Slow pyrolysis may convert about 30% by mass of the biomass to char, with the remainder being converted mostly to non-condensable vapors. Only a small amount of condensable liquid product is obtained during slow pyrolysis.
Ablative pyrolysis is the process of applying high applied mechanical pressure or centrifugal force to particles of feedstock which are moved on a hot surface having a temperature of about 400° C. or higher. This process has the advantages of more effective heat transfer, minimal use of inert or transport gas, and the use of larger feedstock particles than are typically employed in fluid or circulatory beds. Known systems for ablative pyrolysis include vortex pyrolyzers in which the feedstock particles are accelerated to high speeds by the use of a gas jet. Once the feedstock particles are brought into contact with a curved surface, centrifugal forces ensure that the biomass remains in contact with the surface long enough to effect ablative pyrolysis. However, vortex pyrolysis requires very small particles of feedstock, which is costly and inefficient, because grinding of the feedstock into fine powders requires considerable amounts of energy. In experiments on vortex pyrolyzers reported in the literature, the heated surface was subject to erosion due to the initial point of impingement of the jet being subjected to severe abrasion by the entrained feed material in the jet. In addition, as the feedstock particles interacted with the heated wall, they rapidly lost momentum, which reduced the inertial forces that brought the particles and the wall into contact, and the particles tended to exit the chamber without being fully pyrolyzed. U.S. Patent Application Publication No. 2005/0173237 A1 to Bridgwater et al. teaches an ablative thermolysis reactor, shown in FIG. 1, comprising a cylindrical reaction vessel 12 having a heated sidewall 10 and at least one rotatable surface 11 connected with a drum 13 disposed within the cylindrical vessel having an axis of rotation coincident with the longitudinal axis of the cylindrical reaction vessel wherein the rotatable surface is positioned relative to the ablative surface, i.e. the cylindrical reaction vessel side wall, such that the feedstock is pressed against the ablative surface and moved along the ablative surface by the rotatable surface to thermolyse the feedstock. The Bridgwater et al. publication further teaches that the distance between the rotatable surface 11 and the heated side wall, although adjustable, is less than 1 mm. It will be appreciated by those skilled in the art that pressing of the feedstock against the heated side wall corresponds with the application of a radial force upon the feedstock as indicated by arrow 14. Although this approach addresses the issue of maintaining contact between the feedstock particles and the ablative surface, there exists the potential for a buildup of particles between the rotatable surface and the ablative surface which, in addition to preventing a portion of the particles from ever contacting the ablative surface, could lead to overload or jamming of the device. Furthermore, because the apparatus relies upon maintaining continuous pressing of the particles against the ablative surface as the rotatable surface is rotated, the range of particles which may be processed during any single period of operation is limited by the distance between the rotatable surface and the ablative surface, i.e. less than 1 mm. Thus, adjustment of the distance between the rotatable surface and the ablative surface is necessary to accommodate increases or decreases in feedstock particle sizes. In addition, in one embodiment of the reactor of the Bridgwater et al. publication in which the cylindrical reaction vessel is vertically oriented, input of the feedstock into the vessel is through one or more openings in the side wall as is output of the vapor produced by the ablative pyrolysis. One of the drawbacks of this arrangement is the tendency of unreacted feedstock particles, which are being pressed against the heated side wall, to pass through the outlet opening(s) before having been completely pyrolyzed.